1. Field of the Invention
This invention is directed to a three phase motor conversion and monitoring circuit, and more particularly to a monitoring and conversion circuit for converting a three phase AC electrical motor to run on single phase power. The inventive circuit switches a starting capacitor across two inputs of the three phase motor via a normally closed relay contact. The circuit includes a power supply connected between a first pair of motor inputs which powers a voltage sensor connected between a different pair of motor inputs and a phase sequencing detector, all of which must sense normal motor operating conditions prior to switching the starting capacitor out of the motor circuit.
2. Description of the Related Art
It is well known that three phase electrical motors can be operated via single phase line power if the motor has one input connected to each of the line leads and a third input connected to a floating reference point which typically stabilizes at a voltage approximately between the two line leads. During periods of start-up, when large currents are needed to drive the motor, a starting capacitor is temporarily connected from one of the line leads to the floating lead via a normally closed contact until the motor is brought up to normal running speed. This has the effect of providing a large current to the floating lead during start-up, thus greatly increasing motor torque. Typically such starting capacitors are connected via a single phase relay or timing device which monitors the voltage between one of the line leads and the floating lead to sense a voltage which indicates normal motor running speed and then opens the normally closed contact.
A number of problems occur with this typical prior art approach. The monitoring of the voltage across two of the motor inputs may work well for one motor design and not for another motor design, even of identical horsepower. This is due to the fact that differing motor designs have different internal resistances. In motors with higher internal resistances, initial voltages across the monitored inputs can jump to a value greater than needed to trip the monitoring relay, thus prematurely tripping of the relay, resulting in premature trip out of the starting capacitor. Furthermore, the use of a run capacitor in parallel with the start capacitor can cause the monitored voltage to remain high for several seconds even after motor shut down, thus not allowing the start capacitor to be reinserted in the circuit until the run capacitor's voltage was bled off. Also, the run capacitor can cause relay chatter since voltage across the relay can oscillate back and forth around the critical voltage. In addition, the monitoring of a single voltage value means that, when a monitored motor is "plugged" (instantly reversed under load), the motor will still generate a voltage across the monitored branch which is sufficient to maintain the relay contact open. This is an undesirable condition, since a plugged motor needs to be reversed and, if the starting capacitor does not become reconnected, the motor direction cannot be reversed. The lack of hysterisis in the monitoring relays of prior art circuits effectively limited the voltage operating range of such circuits without modification. Often, in prior art circuits, voltage spikes across the monitored motor inputs occurring during motor start-up would cause instantaneous trip-out of the monitoring relay and thus premature drop-out of the starting capacitor. Finally, in prior art circuits, the value of starting capacitors was limited to approximately 100 .mu.FD per motor horsepower before reaching a level at which false tripping of the relay would occur. Some applications, such as high speed lathes driven by relatively low horsepower motors, need larger starting capacitors than this.
It is clear then, that an improved starting and monitoring circuit is needed for converting three phase electrical motors to run on single phase power. Such a circuit should be universally applicable to virtually all three phase motor designs and should monitor multiple voltages and phase sequences such that normal motor operation is assured prior to drop out of the starting capacitor. Such a circuit should allow use of starting capacitors up to 200 .mu.FD per horsepower without false tripping of the switch and should eliminate relay chatter, even when a run capacitor is used.